Snow Leaopard

I have tried loading the new operating system but get the attached message.
Have followed all instructions including reinstalling backup but still the message appears. HOW DO i change the volume to GUID Partition if it wont let me?

In disk utility.
What Mac are you trying to install onto, and are you using the correct generic disc?

What do you mean "it won't let me"?

What happens when you follow the instructions in that pic and select the GUID partition scheme?

And what Mac are trying to install SL on?

You could have provided a little more details.
My guess:
You bought a new disk, installed it on your machine and are trying to install Snow Leopard.
Most disk you buy today are pre-formated for Windows - you have to fix that.
You have to use the Disk Utilities program to create a partition for the Mac OS Extended (Journaling) and make sure you select the "option" button - there you will probably find that "Master Boot Record" is checked.
Check the "GUID" button.
Now commit the new partition and you should be fine.